Embark’s mission is to end preventable disease in dogs. As part of this work, we are committed to providing expert guidance to breeders about genetic health conditions and how the risk of developing them can differ between variants and among breeds. One such group of conditions is progressive retinal atrophy (PRA). Embark currently tests for over a dozen variants related to PRA and actively works with breed clubs and purebred owners to lessen the probability of producing at-risk dogs.
What is PRA in dogs?
Progressive retinal atrophy describes a group of inherited degenerative or dysplastic disorders of the photoreceptor cells of the retina that results in vision loss in dogs (as well as humans). The retina is a complex organ, and even small DNA changes can cause vision loss due to retinal atrophy; this is one of the reasons that there are so many different variants that can cause clinical PRA.
Vision loss in dogs can be caused by a variety of conditions, including PRA, SARDS, retinal detachment, cataracts, glaucoma, trauma, tumors, and infections. These conditions are commonly seen in dogs. PRA occurs in many breeds, and except for certain X-linked disorders, there is no sex predilection. Fortunately, PRA is a non-painful condition, regardless of the causative variant and breed.
Causes of vision loss in progressive retinal atrophy
The retina perceives images through photoreceptors that collect information about light. There are two types of photoreceptors in the retina: rods and cones.
Rods gather information about light intensity and are used more for night vision. Rods are also responsible for detecting and following movement. Cones distinguish between colors and are more helpful for day vision and acute focal vision. Most domestic animals have a dominance of rods. In PRA, the rods degenerate first, so night vision will wane before day vision. Conditions where the cones are affected first (or alone) are more rare and are often referred to as day blindness, cone degeneration, cone-rod dystrophy, or achromatopsia.
As previously mentioned, Embark tests for over a dozen different variants that are known to cause PRA in one or more breeds. Most of these variants are recessive, meaning a dog must inherit a copy of the variant from both the sire and dam to be at-risk from the variant. The variant described in English Mastiffs and Bullmastiffs is dominant, meaning only one copy is needed to confer risk.
Additionally, Embark tests for genetic variants known to cause retinal abnormalities that are either not classified as PRA or have other systemic implications. These conditions include Collie Eye Anomaly (CEA), Canine Multifocal Retinopathy (CMR), Congenital Stationary Night Blindness, and Oculoskeletal Dysplasia 1 (OSD1).
How veterinarians diagnose PRA in dogs
Ocular exams, including evaluation of the retinas, are part of the comprehensive breeding and health management protocols recommended by many breed organizations, and may be required to obtain a CHIC number.
A diagnosis of PRA is made by examination of the fundus, or back of the eye. In early stages, it may be difficult to observe any obvious changes to the retina, but with disease progression, a veterinarian will observe increased reflectivity of a portion of the retina called the ‘tapetum lucidum’, thinning of the retinal blood vessels, and atrophy of the optic nerve. Changes to the fundus are bilateral and symmetrical, helping to distinguish PRA from other retinal diseases.
If the retinas are unable to be evaluated due to other abnormalities (cataracts, corneal scarring, etc.), a veterinary ophthalmologist can perform an electroretinogram (ERG), which measures the electrical activity, and thus the function, of the retinas. Since cataracts often develop secondary to PRA, an ERG is crucial to evaluate for PRA prior to considering cataract surgery.
Clinical signs of reduced vision
Signs that a dog might have decreased vision include:
- Reluctance to go down stairs
- Reluctance to go into a dark room or outside at night
- Bumping into door frames or corners
- Difficulty fetching toys
- A characteristic eyeshine due to increased reﬂectivity of the tapetum
Remember, there are many causes of decreased vision, and a veterinarian can provide more insight after a thorough ocular examination.
While not an official diagnosis, a DNA test can determine if a dog is a carrier or at-risk for multiple forms of progressive retinal atrophy. The Embark for Breeders test kit screens for PRA (along with other breed-relevant genetic health conditions).
Some forms of PRA have no known genetic variant, which means they can not be tested. It is possible, therefore, that a dog who tested clear for the known PRA genetic variants could develop one of these other unidentified forms. This also means that a dog diagnosed with PRA by a veterinarian may still test as clear for all of the known genetic variants. Additionally, some forms of PRA have incomplete penetrance or a breed-specific impact. One such form is crd4/cord1 PRA (RPGRIP1), whose variant is present in many breeds of dogs but is only currently reported to cause clinical PRA in a handful.
Penetrance and breed-specific impact are the focus of on-going research by Embark. Additionally, Embark’s scientists continually work to validate newly published genetic health variants and add them to our DNA Test products. Breeders and all dog owners can support both efforts by completing the surveys found on your dog’s Embark profile. You can also learn more about our research team and projects here.
When do dogs start to show signs of PRA?
Discernible visual impairment will typically lag behind changes observed by a veterinarian and ERG abnormalities. The age at which signs of visual impairment begin to show depends on which variant of PRA a dog has. This distinction will determine if the photoreceptors never formed properly in the first place (photoreceptor dysplasias) or whether they formed properly but then later degenerated (photoreceptor degenerative disorders). Dysplasias affect young dogs and quickly lead to blindness as the photoreceptors never develop normally. In the most common type of PRA, the photoreceptors develop and function normally at first, but then begin to deteriorate later in life. Some degenerative forms of PRA begin in early adulthood and others develop when the dogs are more mature. You can learn more about the PRA variant(s) relevant to your breed by visiting https://embarkvet.com/breeders/health.
Treatment for PRA in dogs
Currently, there is no treatment for progressive retinal atrophy in dogs; however, continuing research on gene therapy may provide some hope for dogs in the future. As dogs can rely on their senses of smell and hearing to make their way through the world, and because the condition is progressive, dogs will adapt to the gradual vision loss over time. There are steps an owner can take to help a dog adjust to being vision-impaired, such as keeping furniture in the same place. This will allow a dog to navigate their home environment based on memory. Any visually-impaired dog should be kept on a leash in unfamiliar environments, especially around other dogs they are not familiar with. Ideally, visually impaired dogs should be trained with verbal commands instead of hand signals.
Cataracts secondary to PRA generally occur later in the disease progression, and are presumed to occur due to oxidative stress on the lens from the degenerating retinas (“toxic” cataracts). Oral antioxidant therapy has been shown to improve retinal function in normal dogs as well as decrease oxidative stress on lens cells, which can help delay cataract formation. Please speak with your veterinarian about which product may be beneficial to an at-risk dog and if treatment should be initiated.
Although dogs who are visually impaired due to cataracts can usually be treated with surgery, removal of cataracts in a dog with progressive retinal atrophy is not recommended. This is because the vision loss is primarily due to retinal degeneration, not the cataracts. Cataracts themselves are not painful, however, chronic advanced cataracts can lead to inflammation within the eyes. If left untreated, this inflammation can lead to increased intraocular pressure and pain (glaucoma). Your vet can help determine a proper monitoring protocol and discuss signs to watch for that may indicate glaucoma.
Embark test results inform breeding decisions
Responsible breeders are testing their dogs for the potential risk of developing genetic health conditions, and this includes the many forms of PRA discussed above. Most PRA variants are recessive, and in these cases breeding a clear dog to a heterozygous (or carrier) dog will statistically result in half the puppies being clear, and half being carriers with no at-risk puppies. Breeding carrier and at-risk dogs is an important consideration in many breeding programs and must be done with an understanding of the genetics involved. Please see this video by Embark’s Chief Science Officer Dr. Adam Boyko for specific examples and guidance on this important topic, and learn more about the importance of annual canine eye screenings.